Fluorescent substances are applied to a vacuum fluorescent display (VFD), a field emission display (FED), a plasma display panel (PDP), a cathode ray tube (CRT), a white light-emitting diode (LED), and the like. In any application of these, in order to emit light from a fluorescent substance, energy for exciting the fluorescent substance has to be given to the fluorescent substance. The fluorescent substance emits visible light by being excited with a high energy excitation source such as vacuum ultraviolet rays, ultraviolet rays, electron beams, and blue light.
However, fluorescent substances have a problem in that, as a result of exposure to the above-mentioned excitation source, the brightness of the fluorescent substances is lowered. Therefore, there is a demand for a fluorescent substance whose brightness will not be lowered. For this reason, sialon fluorescent substances have been proposed as fluorescent substances whose brightness is less lowered, for substituting for conventional fluorescent substances such as silicate fluorescent substances, phosphate fluorescent substances, aluminate fluorescent substances, and sulfide fluorescent substances.
Such a sialon fluorescent substance is produced by a production process as schematically described below.
First, silicon nitride (Si3N4), aluminum nitride (AlN), calcium carbonate (CaCO3), and europium oxide (Eu2O3) are mixed at a predetermined mole ratio. Then, the mixture is held at a temperature of 1700° C. in a nitrogen atmosphere of 1 atm (0.1 MPa) for 1 hour, and calcined by means of a hot press method (see Patent Document 1, for example). It has been reported that the α-sialon in which Eu ion is solid-dissolved, obtained by this process, serves as a fluorescent substance which emits yellow light of 550 to 600 nm by being excited with blue light of 450 to 500 nm, and can be suitably used for the application to a white LED which is produced by a combination of a blue LED and a fluorescent substance. However, there is still a demand for a fluorescent substance which exhibits much higher brightness.
On the other hand, separately from the Ca-containing sialon fluorescent substance described in Patent Document 1, Sr-containing sialon fluorescent substances have also been reported (see Patent Documents 2 and 3).
Of these, Patent Document 2 discloses a sialon fluorescent substance comprising Ca, either alone or in combination with at least Sr or Mg, in which, particularly, the proportion of Sr and/or Mg relative to Ca is preferably 40 mole % at most. It is also described that a sialon fluorescent substance comprising Sr alone serves as a blue-green light-emitting fluorescent substance having an emission peak wavelength of 512 nm.
Patent Document 3 discloses a sialon fluorescent substance which comprises Ca and Sr and has a peak emission wavelength within the range of 600 to 650 nm.
As for the prior arts of illumination devices, white light-emitting diodes made by a combination of a blue light-emitting diode element and a yellow light-emitting fluorescent substance which has a blue light absorption property are known and placed to practical use for the application to a variety of illuminations. Representative examples thereof can include “Light-emitting diode” of Japanese Patent No. 2900928 (Patent Document 4), “Light-emitting diode” of Japanese Patent No. 2927279 (Patent Document 5), and “Wavelength-converting casting material and its manufacturing method and light-emitting device” of Japanese Patent No. 3364229 (Patent Document 6).
In these light-emitting diodes, fluorescent substances particularly used are cerium-activated yttrium/aluminum/garnet based fluorescent substances represented by a general formula (Y, Gd)3(Al, Ga)5O12:Ce3+.
Such illumination devices can be produced by a publicly known method such as those described in Patent Document 7 and Patent Document 8.    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2002-363554    Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2003-124527    Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2006-137902    Patent Document 4: Japanese Patent No. 2900928    Patent Document 5: Japanese Patent No. 2927279    Patent Document 6: Japanese Patent No. 3364229    Patent Document 7: Japanese Unexamined Patent Application, First Publication No. H5-152609    Patent Document 8: Japanese Unexamined Patent Application, First Publication No. H7-99345